P0306 on 2016-2019 Lexus RX HYBRID: Cylinder 6 Misfire Causes and Fixes

What's Unique About the 2016-2019 Lexus RX HYBRID

For this specific generation of the RX 450h, Lexus has issued a Technical Service Bulletin (L-SB-0044-19) that directly addresses misfire codes, including P0306. The bulletin acknowledges that a faulty ignition coil combined with outdated engine control module (ECM) software can cause this issue. The official fix involves replacing the failed coil and reprogramming the ECM with an updated calibration file 🎬 Watch: Official TSB repair procedure for RX450h misfire codes (e.g., for a 2016 model, updating to 348Q4100 or higher). This repair is covered under the Lexus Federal Emission Warranty for 96 months or 80,000 miles.

Diagnostic Flowchart

Tap your situation to follow the diagnostic path that matches what you're seeing on this vehicle.

Symptoms You May Notice

• Check Engine Light is on (may be flashing if the misfire is severe)
• VSC (Vehicle Stability Control) light is on
• Rough or shaky idle
• Hesitation or stumbling during acceleration, sometimes only under heavy load
• Noticeable loss of engine power
• Rough running on initial start-up, especially after a cold soak [Bulletin #L-SB-0045-12 Rev]

Most Likely Causes

1. Faulty Ignition Coil 🔴 High Probability Lexus issued a Technical Service Bulletin (L-SB-0044-19) for this issue, indicating a known problem with ignition coils on this model. The repair procedure in the TSB involves replacing the coil and reprogramming the ECM, confirming this is a recognized failure pattern. Forum users frequently confirm the coil is the culprit.
   How to confirm: Swap the ignition coil from cylinder 6 with another cylinder on the same bank (e.g., cylinder 4, which is also on the front bank and easily accessible). Clear the codes and drive the vehicle. If the code changes to P0304, the ignition coil is confirmed to be faulty. 🎬 Watch: Troubleshooting a P0306 misfire on the 2GR engine
   Typical fix: Replace the faulty ignition coil. Per TSB L-SB-0044-19, the ECM should also be reprogrammed by a dealer or qualified shop with Toyota's Techstream software to prevent recurrence.
   Est. part cost: $70-$150
2. Worn or Fouled Spark Plug 🟡 Medium Probability Spark plugs are a regular maintenance item with a recommended replacement interval. While durable, they can fail, especially if the maintenance schedule is not followed.
   How to confirm: After confirming the coil is good, remove the spark plug from cylinder 6. Inspect it for wear (worn electrode), damage (cracked porcelain), or heavy deposits of carbon or oil. A wet appearance can indicate a lack of spark. Compare it to a new plug.
   Typical fix: Replace the spark plug. It is best practice to replace all six spark plugs at the same time to maintain even engine performance, though accessing the rear three plugs is labor-intensive as it requires removing the intake manifold.
   Est. part cost: $15-$30 per plug
3. Clogged or Faulty Fuel Injector ⚪ Low Probability
   How to confirm: If the coil and spark plug are good, the fuel injector is the next suspect. The 2GR-FXS engine uses Toyota's D-4S system with both port and direct injectors, complicating diagnosis. A mechanic can perform a fuel injector balance test or check its resistance with a multimeter. Swapping injectors is possible but much more labor-intensive than swapping coils.
   Typical fix: Replace the faulty fuel injector. This is less common than ignition system faults, but some owners have reported needing injectors replaced around 70k miles.
   Est. part cost: $100-$250

Rare But Worth Checking

• Damaged Ignition Coil Connector: The plastic connector pigtail for the ignition coil can become brittle with age and heat cycles. The locking tab can break, leading to a poor connection and an intermittent misfire. In some cases, the connector itself may need to be replaced.
• Vacuum Leak: A leak in the intake manifold gasket or a vacuum hose near cylinder 6 can lean out the air-fuel mixture and cause a misfire. This is less common but worth checking with a smoke test if ignition and fuel components are good.
• Low Engine Compression: Internal engine issues like a worn piston ring, a poorly sealing valve, or a blown head gasket can cause a loss of compression, leading to a misfire. This is a worst-case scenario and should be confirmed with a compression test after all other possibilities are exhausted.

Diagnosis Steps

1. Read the trouble codes with an OBD-II scanner to confirm P0306 and any other codes like P0300 or VSC warnings.
2. Note the freeze frame data, which shows engine conditions when the code was set.
3. Locate cylinder 6. On the transverse 2GR-FXS V6 engine, the cylinders are numbered 1-3-5 on the rear bank (against the firewall, difficult to access) and 2-4-6 on the front bank (near the radiator, easy to access). Cylinder 6 is on the front bank, closest to the driver's side. The firing order is 1-2-3-4-5-6.
4. Swap the ignition coil from cylinder 6 with the coil from cylinder 4 (the middle cylinder on the front bank). Inspect the coil's electrical connector for damage or brittleness.
5. Clear the codes and test drive the vehicle until the Check Engine Light returns.
6. If the code is now P0304, the ignition coil is bad and must be replaced.
7. If the code remains P0306, swap the spark plug from cylinder 6 with cylinder 4. Clear codes and retest. If the code moves to P0304, the spark plug is faulty.
8. If the code remains P0306 after swapping the coil and plug, the issue may be a fuel injector, wiring, or a mechanical engine problem requiring further professional diagnosis, such as a compression test or smoke test for vacuum leaks.

Parts You'll Likely Need

• Ignition Coil (OEM #90919-A2013) — This is the most common failure part for P0306 on this vehicle, as documented by Lexus TSB L-SB-0044-19. This part number supersedes 90919-02251, 90919-A2002, 90919-A2004, and 90919-A2007. Denso is the OEM supplier, and it is often possible to buy the Denso-branded part for less than the Toyota/Lexus-branded part.
  Trusted brands: Denso (OEM), NGK
  OEM price range: $90-$150
  Aftermarket price range: $50-$90
• Spark Plug (OEM #90919-01263 (Denso FK20HBR8) or 90919-01247 (Denso FK20HR11)) — A worn or fouled spark plug is the second most likely cause of a misfire. They are a standard maintenance item, recommended for replacement at scheduled intervals.
  Trusted brands: Denso Iridium (OEM), NGK Laser Iridium
  OEM price range: $20-$30
  Aftermarket price range: $15-$25

Related Codes That Often Appear With This One

• P0300 — If the misfire condition is intermittent or affects multiple cylinders, a P0300 (Random/Multiple Cylinder Misfire Detected) may be stored alongside P0306.
• P0301, P0302, P0303, P0304, P0305 — These codes indicate misfires in other cylinders. The presence of multiple misfire codes is specifically mentioned in Lexus TSBs L-SB-0044-19 and L-SB-0045-12 Rev.

Technical Service Bulletins (TSBs) & Recalls

• LSB004419 / L-SB-0044-19: MIL ON With Misfire DTC(s) P0301, P0302, P0303, P0304, P0305, and/or P0306.
• L-SB-0045-12 Rev: MIL 'ON' with DTC P0300/00, P0301/00, P0302/00, P0303/00, P0304/00, P0305/00, and/or P0306/00 present and/or Rough running on initial start-up after a cold soak.

Platform-Specific Known Issues

• Lexus issued Technical Service Bulletin L-SB-0044-19 (revised Feb 2023) for 2016-2019 RX 450h models. It states that a MIL ON with misfire DTCs (P0301-P0306) may occur. The official repair involves replacing the single faulty ignition coil and reprogramming the Engine Control Module (ECM), a job which is allotted 2.8 hours under warranty.
• An earlier TSB, L-SB-0045-12 Rev, also notes that these vehicles may exhibit a MIL ON with misfire codes and rough running on a cold start, indicating a history of misfire sensitivity on this platform.

Mechanic-Grade Diagnostic Values

• Ignition Coil Primary Resistance — expected: 0.49 - 0.57 Ω. Failure: A reading outside this range indicates a faulty coil.
• Ignition Coil Secondary Resistance — expected: 9.5 - 11.1 kΩ. Failure: A reading outside this range indicates a faulty coil.
• Base Ignition Timing — expected: 10° BTDC at idle. Failure: Incorrect timing could point to sensor or ECM issues.
• Coolant Temperature for Diagnostics — expected: Above 176°F (80°C). Failure: N/A. This is a condition required to perform certain active tests in Techstream.

Hidden / Shadow Codes Worth Checking

• Mode $06 Misfire Data: The ECU tracks misfire counts for each cylinder even if they don't reach the threshold to trigger a P030x code. This data can reveal an intermittent or developing misfire before the Check Engine Light is on. A high count on cylinder 6 relative to others points to a problem. (see via Access 'Mode $06' or 'Test Results' on a capable OBD-II scanner. In Toyota's Techstream software, this is found in the 'Misfire Monitor' under the Monitor screen, which shows current 'rate' and historical 'EWMA' counts.)

Scan Tool Commands That Help

• Toyota Techstream: Misfire Monitor — Use this to view detailed current and historical misfire counts per cylinder ('rate' and 'EWMA'). This is more accurate than the live data list for diagnosing intermittent misfires that occur under load, as the counts are recorded even when not actively viewing the screen.
• Toyota Techstream: Control the Injection Mode (Active Test) — For the D-4S (2GR-FKS) engine, this test allows a technician to manually switch between 'Port' and 'Direct' injection. This can help isolate whether a misfire is related to a specific set of injectors (port vs. direct) after ignition issues have been ruled out.
• Toyota Techstream: ECM Reprogramming (Calibration Update) — This is a required step when replacing an ignition coil for a misfire code, per TSB L-SB-0044-19. The software update helps prevent the misfire from recurring.

Wiring & Ground Locations

• Ignition Coil Connector — On top of each ignition coil, connected to the engine wiring harness.. The 4-pin connector's plastic locking tab can become brittle from engine heat and break off. This can lead to a loose connection, causing intermittent misfires on cylinder 6. If the tab is broken, the connector housing can be replaced without cutting wires.

Real Owner Repair Stories

• ClubLexus Forum (2007 GS 350 (Similar 2GR engine)) — Persistent P0306 code, Check VSC light on.
  ❌ Tried (didn't work) Replacing spark plug on cylinder 6., Replacing ignition coil on cylinder 6 without diagnosis.
  ✅ What actually fixed it After swapping the new (but faulty) coil and plug didn't work, the owner swapped the known-good coil from cylinder 4 to cylinder 6. The code remained P0306. A compression test was fine. The final cause was identified as significant carbon buildup in the engine, which was cleaned to resolve the misfire.
• ClubLexus Forum (2016 RX 450h) — Rough idle, knocking/rattle noise on cold start.
  ✅ What actually fixed it The issue was diagnosed by the dealer as faulty fuel injectors. The repair was covered under warranty, referencing a Technical Service Bulletin. The owner noted the cost would have been over $2,100 without warranty.

OEM Part Supersession History

• 90919-02280 → 90919-A2013 — Updated part design to address failure rates, as specified in TSB L-SB-0044-19.

Model Year Variations Within This Range

• 2016-2019: The ECM software calibration is specific to the model year. TSB L-SB-0044-19 lists the 'New' calibration ID that must be installed. For a 2016 RX 450h, the new calibration is 348Q4100 or higher. For a 2017, it's 348N0100 or higher. For a 2018, it's 348V6100 or higher. For a 2019, it's 348Z3100 or higher.